Myelosuppression is the most common life-threatening complication of anti-neoplastic therapy. A great deal is known about the activation of hematopoietic stem and progenitor cells (HSPCs) to initiate recovery from myelosuppression. Yet it is not known how these processes wind down to allow hematopoiesis to return to homeostasis. The long-term goal of this research is to develop new approaches for treating multi-lineage myelosuppression. The objective of this application is to identify the signaling pathways that check recovery and reinstate homeostasis during hematopoietic regeneration after myelotoxic chemotherapy. The central hypothesis is that TGF ? signaling is activated during recovery from myelotoxic stress, and this induces a transcriptional response in HSPCs that re-establishes their quiescence and dampens hematopoietic regeneration. The rationale for the proposed studies is that, once we understand how homeostasis is restored after hematopoietic stress, we can pharmacologically manipulate the recovery to limit chemotherapy-induced myelosuppression and allow more efficacious treatments to be delivered safely. Three specific aims will be pursued to test the hypothesis. Aim 1: Identify how TGF ? signaling is activated during recovery from myelotoxic stress. Aim 2: Define how TGF ? target genes in HSPCs re-establish homeostasis during stress recovery. Aim 3: Determine how TGF ? blockade can be safely combined with chemotherapy to minimize myelosuppression. All three aims are well supported by preliminary studies and use methodologies that have already been established to be feasible in the applicants' hands. The first aim will identify the bone marrow cells from which TGF ? originates and determine how latent TGF ? is locally activated during recovery from myelotoxic therapy. The second aim will identify the downstream signaling pathways that mediate TGF ?-enforced restoration of HSPC homeostasis following chemotherapy. With the third aim, well-controlled pre-clinical studies will assess the long-term safety and potential efficacy of combining TGF ? pathway inhibitors with chemotherapy as a strategy to minimize myelosuppression. Currently, TGF ? pathway inhibitors are being clinically developed for their direct anti-neoplastic activities. Ultimately, the proposed studies are expected to show that TGF ? blockade after chemotherapy can limit myelosuppression while it is augmenting tumor cell kill: a new double-edged sword to attack cancer. This contribution is significant because it opens the door to new classes of agents that promote multi-lineage hematopoietic regeneration by modulating the return of HSPCs to steady-state quiescence. The proposed research is innovative because it is a substantial departure from the status quo and challenges the de facto paradigm that homeostasis is passively reestablished as cytokine levels normalize after stress. This work is expected to vertically advance our understanding of the hematopoietic adaptations to stress and it is likely that what is learned here can be extrapolated to other adult stem cell types that are induced in response to tissue damage.